Boston firefighters battled flames and rescued one person Monday morning at a multifamily building in Dorchester. Firefighters said the fire was visible from the exterior when they arrived around 6:25 a.m. A second alarm was struck to call for additional resources. Officials described burning structure at 83 Holmes Ave. as a 3-family occupied building. The injured person was found on the third floor, where officials also said the fire started.The injured person was taken to Boston Medical Center. Their condition is unknown. Officials also said they believe one dog perished in the fire.